[X86][PREFETCH]: Adding full coverage of MC encoding for the PREFETCH isa sets.<NFC>

[X86][PREFETCH]: Adding full coverage of MC encoding for the PREFETCH isa sets.<NFC>

NFC.
Adding MC regressions tests to cover the PREFETCH isa sets for both 32 and 64 bit.
This patch is part of a larger task to cover MC encoding of all X86 ISA Sets started in revision: https://reviews.llvm.org/D39952

Reviewers: zvi, craig.topper, RKSimon, AndreiGrischenko
Differential Revision: https://reviews.llvm.org/D41161

Change-Id: Icdc8c5fb68c414de7d2cfdb50da1cc6763d9932a

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@321524 91177308-0d34-0410-b5e6-96231b3b80d8